With the development of science and technology, dyes are closely related to human daily life. Today, synthetic dyes are widely used in food, printing and dyeing, cosmetics and pharmaceutical industries. Throughout the world, about 12% of the dyes used are lost in their processing and handling operations. Of which, 20% pass through wastewater into the environment, resulting in pollution of the water environment. It can be seen that dye wastewater seriously endangers the water environment. These dyes are usually biologically toxic and carcinogenic, and difficult to degrade by microorganisms in the environment. Therefore, the search for cheap, efficient and energy-saving methods to degrade dye wastewater has become a hot issue in environmental research.
Since Honda et al. used TiO2 electrodes for hydrogen production by photolysis of water in 1972, the research on semiconductor photocatalysts has developed rapidly. Photocatalysis refers to that by light irradiation, semiconductor generate carriers and undergoes the separation of photogenerated carriers. And then photogenerated electrons and holes combine with ions or molecules to produce reactive radicals with oxidative or reductive properties. These reactive radicals can degrade organic macromolecules into carbon dioxide and water. Therefore, the photocatalytic technology has the advantages of high efficiency and low cost.
Nowadays, the research on photocatalytic performance of ZnO, TiO2, CdS and SnO2 is becoming more and more important. Among them, titanium dioxide has many advantages, such as good photocatalytic activity, high stability, acid and alkali resistance, non-toxic to the biological, rich sources and low cost, so it is the most frequently used for photocatalytic research. However, the application of titanium dioxide in the field of photocatalysis has been limited by the following factors: 1), the recombination of photo-generated electrons and holes leads to low quantum yield. 2), TiO2 has a wide band gap of 3.2 eV and exhibits photocatalytic activity under ultraviolet excitation. 3), the mass transfer rate between TiO2 and pollutant is low.